Wavelength Division Multiplexing is a well known method of increasing fibre capacity by allocating different client signals to different optical carrier wavelengths. A special class of fibre access communication system exploits bidirectional communication, e.g. between end user and Central Office, on a single fibre and using the same wavelength in both directions in order to maximize the carried capacity. This type of transmission is generally referred as wavelength reuse and needs special kinds of modulation formats to mitigate cross-talk between the two propagation directions. The use of special modulation formats (e.g. IRZ/RZ) can mitigate but not cancel out the cross-talk terms due to the optical reflections along an optical fibre link and residual downstream modulation on the upstream traffic.
An alternative way to increase fibre capacity is to exploit different propagation modes. Multi-mode fibres have been on the market for a long time but optical signals propagating in multi-mode fibre suffer inter-modal dispersion and intensity noise, which cause a significant transmission penalty, and they have therefore never really been exploited for optical communications over long distances. Recently however, mode-division multiplexed, MDM, wavelength division multiplexed, WDM, transmission has been reported, e.g. by N. Bai et al ‘Mode-division multiplexed transmission with inline few-mode fiber amplifier’, Optics Express, Vol. 30, No. 2, 30 Jan. 2012.